In addition to the pressure balance given in Equation 7-73 and consideration for mass flux across the interface in Equation 7-74 and Equation 7-75, the Beavers–Joseph slip condition models the development of the boundary layer at the intersection of the physics interfaces using a small interface slip velocity.

The condition ensures that the tangential velocity at the interface is continuous when the slip velocity is included at the wall and base boundaries of the thin-film region. The slip length parameter accounts for the slip velocity, as given in Equation 9-9 in the CFD Module Reference Manual. Depending on the orientation of the interface normal with respect to the reference surface normal, the expressions are given as

Here, tr() is the trace operator, κ is the permeability tensor of the porous media, v is the thin-film velocity, subscripts b and w represent the base and wall boundaries of the thin-film region, respectively, and the subscript t represents the component tangential to the interface.